The present invention relates to an optical access system for providing communication between a subscriber's home and a communication common carrier's office and more particularly to an optical access system using an error correcting code.
Public telecommunication networks for transferring data such as voice and images include an access network for accommodating users to offices. A telephone subscriber network and ADSL have been used as such access network. In recent years, an optical access system also began to be introduced.
Some optical access systems provide one-to-one connection between an office and a subscriber while others provide one-to-n connection therebetween. A PON (Passive Optical Network) optical access system is known as the optical access system for one-to-n communication.
The PON optical access system allocates one optical wavelength to each of upstream and downstream. The system provides data communication between an optical line terminal (OLT) and multiple optical network terminals (ONTs) while sharing a band. A downstream signal from an OLT 1 to an ONT 2 passes through an intermediate optical splitter that distributes an optical signal. The ONT 2 acquires only a signal addressed to itself for communication. The OLT 1 notifies the ONT 2 of timing to transmit an upstream signal. The ONT 2 transmits the signal to the OLT 1 at that timing. In this manner, multiple ONT 2 terminals share one wavelength to communicate with the OLT 1.
Such known optical access systems include B-PON (Broadband PON, see ITU-T Recommendation G.983.1, G.983.4), GE-PON (Gigabit Ethernet PON, see IEEE IEEE802.3ah), and G-PON (Gigabit capable PON, see ITU-T Recommendation G.984.1, G.984.4).
The PON system accommodate such signals as noncyclic signals flowing through the Internet such as Web and mail, cyclic signals communicated through telephone networks or leased line networks, and signals at approximately constant bit rates such as video signals.
With reference to FIG. 1, the following describes an optical access system according to the related art. As shown in FIG. 1, an optical access network is configured between an OLT 1A, an ONT 2A-1, and an ONT 2A-2. Each ONT 2A and the OLT 1A are connected via an optical splitter 3. The ONT 2A is connected to an IP system 4. The OLT 1A is connected to an IP network 5. The IP network 5 accommodates a signal from the IP system 4 via the optical access network.
The splitter 3 and the ONT 2A are installed in a house or a building. A distance to the house or the building depends on the ONT 2A. That is, a distance between the OLT 1A and the ONT 2A depends on each ONT 2A within an allowable range. To solve this problem, the OLT 1A performs a ranging process to measure a distance to the ONT 2A and corrects an upstream signal phase. When the OLT 1A starts the ranging process, each ONT 2A immediately returns a signal to perform the ranging process. The OLT 1A corrects the phase assuming that the response time (delay time) from each ONT 2A corresponds to the distance. Using the phase correction, the OLT 1A ensures consistency between the time recorded in itself and the time recorded in the ONT 2A based on respective distance differences. Accordingly, the OLT 1A and all ONT 2A terminals can share an upstream time slot. The OLT 1A provides each ONT 2A with a communication timing and a band, that is, a start position and a stop position in the upstream time slot. Each ONT 2A transmits a signal to a communication path at the specified timing. This avoids collision between upstream signals transmitted from the ONT 2A terminals.
GE-PON and G-PON use an 8-byte correction Reed-Solomon (255, 239) code as the error correction code or the forward error correction (FEC) code for improving the communication quality.
The technology disclosed in Japanese Patent Laid-Open 2006-014228 selects an FEC code for each ONT 2A in accordance with a communication distance between the OLT 1A and the ONT 2A irrespectively of the Reed-Solomon (255, 239) code. The technology disclosed in Japanese Patent Laid-Open 2008-306434 allocates a band in consideration for FEC redundancy. Japanese Patent Laid-Open 2008-060820 and 2008-148245 disclose OLT and ONT as terminals that can be miniaturized.
With reference to FIG. 2, the following describes an FEC code selected for each ONT in the optical access system. For downstream communication as shown in FIG. 2, the OLT LA communicates with the ONT 2A-1 using FEC code (A), with the ONT 2A-2 using FEC code (B), and with the ONT 2A-3 using FEC code (C). In communication with the ONT 2A, the OLT 1A uses an FEC code having correction capability corresponding to the communication distance or the communication quality. It is assumed that the communication distance between the OLT 1A and the ONT 2A decreases or the original communication quality improves in the order of the ONT 2A-1, the ONT 2A-2, and the ONT 2A-3. The OLT 1A accordingly uses FEC code (A), FEC code (B), and FEC code (C) in the descending order of error correction capabilities. In response, the ONT 2A-1 through the ONT 2A-3 respectively use the allocated FEC code (A), FEC code (B), and FEC code (C) to decode the addressed signals.
For upstream communication as shown in FIG. 2, the ONT 2A-1 uses FEC code (D) for communication with the OLT 1A. Similarly, the ONT 2A-2 uses FEC code (E) for communication with the OLT 1A. The ONT 2A-3 uses FEC code (F) for communication with the OLT 1A. Each ONT 2A communicates with the OLT 1A using an FEC code having correction capability corresponding to the communication distance or the communication quality. It is assumed that the communication distance between the OLT 1A and the ONT 2A decreases or the original communication quality improves in the order of the ONT 2A-1, the ONT 2A-2, and the ONT 2A-3. The ONT 2A terminals accordingly use FEC code (D), FEC code (E), and FEC code (F) in the descending order of error correction capabilities. In response, the OLT 1A uses FEC code (D), FEC code (E), and FEC code (F) to decode signals.
The OLT 1A notifies the ONT 2A of FEC code allocation to each ONT 2A using an overhead for downstream communication, as will be described later. Similarly, the OLT 1A notifies the ONT 2A of upstream communication from the ONT 2A to the OLT 1A using an overhead for downstream communication.
The FEC code is selected in accordance with the communication distance or the communication quality between the OLT 1 and the ONT 2 so as to adjust the error correction capability for each ONT 2. The FEC code redundancy or the number of check bits varies with the error correction capability. The redundancy can be controlled in accordance with the error correction capability needed for each ONT 2 and bands can be used effectively in comparison with a case of allocating the most efficient (most highly redundant) FEC code to all ONT 2 terminals in common. When the FEC code is selected for each ONT 2, however, it is necessary to use multiple FEC codes having different FEC code formats and error correction capabilities. The FEC code formats include Reed-Solomon, BCH, and convolution code and require calculations based on specific algorithms. Increasing the error correction capability of the code also increases the number of FEC check bits (redundancy). As a result, FEC code lengths also differ from each other.
Consequently, the OLT 1 and the ONT 2 need to be provided with multiple hardware components such as FEC encoder/decoder circuits for processing FEC codes. In addition, the OLT 1 and the ONT 2 require the band allocation in consideration for different code lengths and redundancies.
When FEC codes are carefully selected in accordance with the communication distance and the communication quality, many types of hardware are needed so as to minimize unnecessary FEC redundancy and maximize the band usability. The OLT 1 needs to calculate the band allocation to the ONT 2 in consideration for many FEC code combinations.
The band is allocated as follows. The ONT 2 notifies the OLT 1 of the stored amount of upstream data. The OLT 1 calculates a bandwidth capable of transmission from the ONT 2. The OLT 1 notifies the ONT 2 of the bandwidth. The ONT 2 and the OLT 1 perform the calculation and the notification as real-time processing. When the calculation also uses the FEC redundancy as a necessary factor, the OLT 1 requires more complicated calculation for more combinations of FEC codes. This delays the response time for the band allocation.
Installing many types of hardware such as different FEC encoder/decoder circuits increases prices of the OLT 1 and the ONT 2. The hardware scale increases as many FEC codes with high correction capabilities are used. This also increases prices of the OLT 1 and the ONT 2.
The optical splitter splits downstream PON signals. When the FEC code is selected for each ONT 2, multicast (or broadcast) may be used to transmit the same signal to multiple ONT 2 terminals and an overhead is transmitted to all ONT 2 terminals in common. Such multicast and overhead are inappropriate for FEC processing. For example, let us suppose that FEC (C) selected for the ONT 2-3 is used to encode and multicast data. The ONT 2-3 can correctly decode the data. However, the ONT 2-1 cannot correctly decode the data because FEC (A) is selected for the ONT 2-1. On the other hand, the band usability degrades when the same data is encoded with FEC (A) for ONT 2-1 and with FEC (C) for the ONT 2-3 and both data are transmitted over a downstream signal. That is, the effect of selecting the FEC code for each ONT 2 decreases.